1. Field of Technology
The present invention relates to image forming apparatuses, image forming methods, and image forming programs, and in particular, to image forming apparatuses, image forming methods, and image forming programs for forming images using distributions of two or more types of dots with different densities per unit area in the same hue.
2. Description of the Related Technology
Conventionally, printers using the three colors of cyan (C), magenta (M), and yellow (Y), or, printers using inks of four colors CMYK with ink of black (K) color added to said three colors have been known as the printing apparatuses that print images from a personal computer, etc. Such printers form images by the ON/OFF control of dots of the respective colors obtained by halftoning.
In contrast with this, in recent years, in order to obtain high image quality output images, many printers have been introduced that carry out printing using inks of different densities (e.g. dark colored ink and light colored ink) are used or different ink dot diameters are utilized so that a dot having a plurality of gradation levels in one hue is formed.
However, one of the typical methods of halftoning is a dither method. This method is one that establishes a one to one correspondence between the pixel value of the input image and one pixel of the threshold value matrix, and the output signal is made ON (the dot is formed) if the pixel value of the input image is higher than the threshold value of the threshold value matrix, and the output signal is made OFF (the dot is not formed) if the threshold value of the threshold value matrix is higher.
When the number of pixels constituting this threshold value matrix is m×n and there are t number of types of threshold values allocated to them, in the case of a common threshold value matrix, very often (m×n)/t of the respective threshold values will be present. For example, if the number of types of threshold values allocated to a pixel of a threshold value matrix of 64×64 pixels is 256 types (when the threshold value matrix has an 8-bit resolution), each value will be present 16 times.
Although this method is extremely fast and does not generate pseudo-outlines in the case of judging the presence or absence of the formation of one type of dot with one type of input value and one type of threshold value matrix, in the case of judging the presence or absence of the formation of two or more types of dots with one type of input value and one type of threshold value matrix (see, for example, Patent Document 1), there was the problem that, at a dot rate of realizing at least one type of dot, discontinuous points will be generated, and as a result, pseudo-outlines were easy to be generated.
In view of this, methods are being thought of for preventing the generation of pseudo-outline in the case of judging the presence or absence of the formation of two or more types of dots having different densities per unit area, and different methods have been proposed.
Incidentally, “dots having different densities per unit area” includes in addition to the dots having different optical densities such as dark dot, mid dark dot and light dots, the dots having different dot sizes such as large dot, middle dot and small dot, and combinations of different optical density dots and different sized dots such as dark dot and small dot, or large dot and light dot.
Patent Document 1: Japanese Translation of PCT International Application Publication No. 2003-501300
However, in the conventional methods that have been proposed for preventing the generation of pseudo-outlines, it is not possible to fully correct pseudo-outlines, and the measures were insufficient.